Positioning systems in high speed printers for data processing systems include a paper carriage mechanism driven by a dc motor operated by electronic controls to feed paper one or more print line spaces at a time relative to a print line defined by impression devices such as print hammers. The positioning operation consists of accelerating the motor quickly to a selected velocity, maintaining the velocity constant until a selected position is reached, and then decelerating the motor to a desired stop or detent position whereupon the motor is deenergized and printing can commence. The electronic controls might include a microprocessor or other controller and a position counter which counts displacement signals generated by an optical or other type encoder device coupled to the motor and/or the carriage drive shaft. The optical encoder device typically comprises a pair of light sensors and an opaque disk having one or more tracks with uniformly spaced transparent indicia. The sensors, the disk, and a light source are arranged so that a pair of motion signals are generated in phase quadrature in response to motor rotation. Decode or other circuitry converts the phased signals into a direction signal and displacement pulses where each pulse indicates movement of a fraction of a line space. The microprocessor, in addition to other control functions involving the motor and other devices associated with the printer, sets the position counter prior to and during motor operation to count the displacement pulses to effect the sequence of motor control operations described.
The rapid starting and stopping of the carriage mechanism and associated mechanisms produces mechanical perturbations which cause undesired movement of the carriage mechanism, the motor, and the paper. The direction and magnitude of this drift movement is essentially variable and irregular. Unlike a stepper motor, the dc motor has no holding torque opposing this undesired movement. Consequently, the motor is periodically operated during printing to reposition the paper. During printing, the electronics control has ample time to detect the direction and magnitude of the undesired movement and to operate the motor to restore the paper to the desired print line. However, in the interval between the end of printing and the start of motor operation, i.e., when the microprocessor is setting the position counter and performing other control functions in preparation for the next move, the microprocessor cannot also operate to monitor and correct for movement since its function is sequential. Therefore, any movement occurring during this time interval results in the loss of carriage position. Consequently, subsequent motor operations result in irregular spacing of the paper causing poor vertical registration due to the non-uniform spacing of the lines of printed data. It is the purpose of this invention to provide an improved control system for a dc motor which solves the problem of drift movement, does not require microprocessor intervention or direct control, and thereby eliminates positioning errors particularly line positioning errors of a print medium in a line printer.